The disclosure relates to a measuring device for measuring a distance between the measuring device and a target object with the aid of optical radiation. Such a measuring device is also designated as an optical distance measuring device and can be provided as a separate, for example handheld, device or in another device, for example a tool. In particular, the disclosure relates to an optical distance measuring device equipped with a calibration unit in order to be able to increase a measurement accuracy of the distance measuring device by calibrating an actual distance measurement.
Optical distance measuring devices are known which align a temporally modulated light beam in the direction toward a target object whose distance from the measuring device is intended to be determined. Returning light reflected or scattered back from the target object aimed at is at least partly detected by the measuring device and used for determining the distance to be measured. In this case, a typical measurement range is for distances from a few centimeters up to several hundred meters.
US 2007/0182949 A1 discloses a distance measuring device comprising a light source for illuminating the target object using continuously modulated light, a solid-state image sensor comprising an array of avalanche photodiodes, and a plurality of circuits for processing signals that have been output by the avalanche photodiodes, in order to provide data which are dependent on the light reflected from the target object onto the photodiodes. The circuits have a multiplexer designed to accumulate signals output by the avalanche photodiodes during different subperiods in different storage devices.
The avalanche photodiodes serving as photon counters in this case receive the light reflected back from the target object and also background radiation additionally present and generate at their output electrical pulses in each case, wherein the temporal pulse density correlates with the impinging light power. The read-out of the pulses from the avalanche photodiodes is effected with the aid of a multiplexer arrangement. The latter can be operated synchronously with a modulation of a laser used as light source in such a way that the pulses of the avalanche photodiodes increment different digital counters in a manner dependent on the point in time of the respective detection events, that is to say for example of a photon absorbed in the avalanche photodiode. A temporal period with which the light source illuminates the target object in a modulated fashion is in this case subdivided into a plurality of subperiods. A number of digital counters corresponding to the number of subperiods are provided, wherein, during each subperiod, a correspondingly assigned digital counter is in each case incremented in accordance with the detection pulses received during the subperiod. In this way, detection events can be accumulated over a measurement time. While an individual period can have, for example, time durations in the range of nanoseconds, the total measurement time can comprise many such periods and last, for example, several milliseconds or several seconds. By accumulating the measurement results in the digital counters, it is possible to record a type of histogram of the detection events relative to the temporal occurrence of detection events within subperiods. As soon as a modulation impressed on the modulated light emitted by the light source is present in the counter readings of the digital counters with sufficient statistical accuracy, it is possible, by means of a phase evaluation, to deduce a propagation time of the light between emission and detection and hence a distance between the distance measuring device and the target object. Such a principle of laser distance measurement is generally known by the designation “time of flight ranging” for example with continuous or pulsed modulation of the intensity of the laser beam.
An evaluation unit which operates in this way and which, within a distance measuring device, receives detection signals from a light-sensitive detector and evaluates them by registering the detection signals in a manner synchronized with a reference, that is to say accumulating them in accordance with their temporal occurrence relative to the periodicity of the modulated measurement light used, is also designated as so-called “binning architecture”. Such a binning architecture can be realized for example with a delay locked delay line (DLL).
It has been observed that distance measuring devices which operate for example in the manner described above on the basis of photon counters, multiplexer arrangements and binning architectures cannot always yield satisfactory measurement accuracies.